This invention relates to the compression testing of materials and in particular to the testing of laminate samples.
It is known to test materials, to establish compression and shear parameters by placing test specimens of the material into a press and compressing them until they fail. An established method of testing a laminate sample of composite materials is to bond strain gauges and clamping surfaces to its surfaces and then to clamp it in a press. The sample is then subjected to a load and compressed until it fails. Deformation of the specimen is detected by the strain gauges and recorded against load so that the behaviour of the material can be analysed under different loads. The ends of the specimen usually have a plastic material bonded to them to prevent the sample failing at the points where it is gripped by the press. Irregularities in the grips of the press often cause localised points of stress which nip the ends of the sample and cause it to fail. Plastic pads smooth out irregularities and spread the load evenly over each end of the specimen.
This method of testing samples is time consuming and costly. Because the strain gauges are bonded to the specimen under test it is not practical to remove them for re-use, consequently a new set of strain gauges is required for each sample to be tested which are then discarded with the specimen after test. Similarly applying uniform layers of plastic to the ends of each specimen is a time consuming and costly business.
Another problem arises when analysing specimens of multi angular laminates which compress to a considerable extent before they begin to fail, consequently very long samples are required for a compression test. The problem is that long strips are flexible and liable to buckle and snap before they have compressed very far. A method of overcoming failure of samples due to buckling is to place a stabiliser around the specimen to prevent it from flexing outwardly and snapping. One form of stabiliser, for example, consists of a solid rod of material with a slot along one side. The sample to be tested has its edge placed inside the slot and is then compressed; buckling of the sample is limited to the width of the slot which is just sufficient to allow failure of the sample due to compression.
A disadvantage of such stabilisers is that they are of a fixed slot width and limited to one particular specimen thickness, so different stabilisers are required for different specimens widths. Another problem encountered with fixed stabiliser arrangements is that the samples frequently jam tight into the stabiliser after failing and they are difficult to remove before inserting the next sample. The stabiliser is sometimes itself a source of failure if for instance the stabiliser should move laterally during compression of the sample it can cause the specimen to kink and sheer at the ends of the stabiliser.
A large number of specially prepared specimens are usually required to test for compression failure because many of the samples will snap due to nipping, buckling or kinking before they have reached the limit of their compressive properties and this increases the expense of the test. It would therefore be an advantage if a system could be implemented to eliminate failures due nipping, buckling or kinking so that specimens could be consistently induced to fail by compression. An object of the present invention is to eliminate such failures and reduce the time and cost of processing each specimen.